Method and apparatus of radiographic imaging with an energy beam tailored for a subject to be scanned

ABSTRACT

A method and apparatus for tailoring the profile of an x-ray beam for radiographic imaging for a specific subject is disclosed. The invention includes a filter assembly having a pair of filters, each of which may be dynamically controlled by a motor assembly during data acquisition. The filters are positionable in the x-ray beam so as to shape the intensity profile of the x-ray beam. In one exemplary embodiment, the filters are dynamically positioned during CT data acquisition based on the shape of the subject. A method of determining the shape of the subject prior to CT data acquisition is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is continuation of and claims priority of U.S.Ser. No. 11/419,851 filed May 23, 2006, which claims priority of U.S.Ser. No. 10/605,789 filed Oct. 27, 2003, now U.S. Pat. No. 7,076,029,the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to diagnostic imaging and, moreparticularly, to a method and apparatus of dynamically filteringradiation emitted toward a subject during radiographic imaging in amanner tailored to the shape and/or position of a subject to be imaged.

Typically, in radiographic imaging systems, an x-ray source emits x-raystoward a subject or object, such as a patient or a piece of luggage.Hereinafter, the terms “subject” and “object” may be interchangeablyused to describe anything capable of being imaged. The beam, after beingattenuated by the subject, impinges upon an array of radiationdetectors. The intensity of the attenuated beam radiation received atthe detector array is typically dependent upon the attenuation of thex-rays. Each detector element of the detector array produces a separateelectrical signal indicative of the attenuated beam received by eachdetector element. The electrical signals are transmitted to a dataprocessing system for analysis which ultimately produces an image.

In computed tomography (CT) imaging systems, the x-ray source and thedetector array are rotated about a gantry within an imaging plane andaround the subject. X-ray sources typically include x-ray tubes, whichemit the x-rays as a beam at a focal point. X-ray detectors typicallyinclude a collimator for collimating x-ray beams received at thedetector, a scintillator for converting x-rays to light energy adjacentthe collimator, and a photodiode for receiving the light energy from anadjacent scintillator and producing electrical signals therefrom.Typically, each scintillator of a scintillator array converts x-rays tolight energy. Each photodiode detects the light energy and generates acorresponding electrical signal. The outputs of the photodiodes are thentransmitted to the data processing system for image reconstruction.

There is increasingly a need to reduce radiation dosage received by apatient during an imaging session. It is generally well known thatsignificant dose reduction may be achieved by using a “bowtie” filter toshape the intensity profile of an x-ray beam. Surface dose reductionsmay be as much as 50% using a bowtie filter. It is also generally knownthat different anatomical regions of a patient may advantageouslymandate different shaped bowtie filters to reduce radiation dosage. Forexample, scanning of the head or a small region of a patient may requirea bowtie filter shaped differently than a filter used during a largebody scanning session. It is therefore desirable to have an imagingsystem with a large number of bowtie filter shapes available to best fiteach patient. However, fashioning an imaging system with a sufficientnumber of bowtie filters to accommodate the idiosyncrasies encounteredduring scanning of numerous patients can be problematic in that eachindividual patient cannot be contemplated. Additionally, manufacturingan imaging system with a multitude of bowtie filters increases theoverall manufacturing cost of the imaging system.

Further, for optimum dose efficiency, i.e. best image quality at thelowest possible dose, the attenuation profile created by the bowtiefilter should be particular to the patient. That is, it is desirable andpreferred that when selecting a pre-patient filter that the patient'ssize, shape, and relative position be taken into account. By taking thepatient's size, shape, and position into consideration, radiationexposure can be tailored to the specific patient. Further, it isgenerally well-known that photon counting (PT) and energy discriminating(ED) CT systems are not possible today, primarily because the largedynamic range of photon flux rates exceeds the count rate capabilitiesof current PT and ED detectors. Tailoring the pre-patient filter to thesubject to be scanned also allows for conforming the filter to minimizephoton flux rates in a range suitable to permit continued development ofPT and ED CT systems. As noted above, the differences in patients in thepotential subject pool are significantly large and fitting a CT systemwith a pre-patient filter for each possible patient profile is more thancost prohibitive; its simply not practical.

Therefore, it would be desirable to design an apparatus and method ofdynamically filtering the radiation emitted toward the subject for dataacquisition in a manner tailored to particular physical characteristicsof the subject. It would be further desirable to have a system thattailors the radiation emitted toward the subject during data acquisitionbased on a scout scan of the subject.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is a directed method and apparatus for defining atailored attenuation profile of a pre-subject, beam shaping filter thatovercomes the aforementioned drawbacks. A filter assembly is providedand has a pair of filters, each of which is dynamically controllable bya motor assembly during data acquisition. The filters may be positionedin the x-ray beam so as to shape the profile of the x-ray beam. In oneexemplary embodiment, the filters are dynamically positioned during CTdata acquisition based on the shape of the subject. A method ofdetermining the shape of the subject prior to CT data acquisition isalso disclosed.

Therefore, in accordance with one aspect of the present invention, abeam shaping filter assembly is provided. The filter assembly includes afirst moveable filter having a non-uniform thickness and a secondmoveable filter independent of the first moveable filter and having anon-uniform thickness. Each filter is configured to be placed in a highfrequency electromagnetic energy beam for attenuation of the beam forradiographic data acquisition.

In accordance with another aspect, a CT system is disclosed thatincludes a rotatable gantry having an opening to receive a subject to bescanned and a high frequency electromagnetic energy projection sourceconfigured to project a high frequency electromagnetic energy beamtoward the subject. A pre-subject filter assembly including a pair offilters and a scintillator array having a plurality of scintillatorcells wherein each cell is configured to detect high frequencyelectromagnetic energy passing through the subject are also provided.The CT system also includes a photodiode array optically coupled to thescintillator array and comprising a plurality of photodiodes configuredto detect light output from a corresponding scintillator cell and a dataacquisition system (DAS) connected to the photodiode array andconfigured to receive the photodiode outputs. An image reconstructor isprovided and connected to the DAS and configured to reconstruct an imageof the subject from the photodiode outputs received by the DAS. The CTsystem further includes a controller configured to independentlyposition each filter of the pair of filters in the high frequencyelectromagnetic energy beam so as to modulate the beam to have a profilethat substantially matches at least an approximate shape of the subject.

According to another aspect, the present invention includes an x-rayfilter assembly having a first filter and a second filter. A first motorassembly is connected to the first filter and a second motor assembly isconnected to the second filter. The first and the second motorassemblies are configured to independently position a respective filterin an x-ray path to define an attenuation profile that substantiallyapproximates a target shape.

Various other features, objects and advantages of the present inventionwill be made apparent from the following detailed description and thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate one preferred embodiment presently contemplatedfor carrying out the invention.

In the drawings:

FIG. 1 is a pictorial view of a CT imaging system.

FIG. 2 is a block schematic diagram of the system illustrated in FIG. 1.

FIG. 3 is a plan view of a representative x-ray system.

FIG. 4 is a sectional view of a portion of the x-ray system shown inFIG. 1.

FIG. 5 is a schematic view of one embodiment of a pre-subject, beamshaping filter assembly in accordance with the present invention.

FIG. 6 is a schematic view of another embodiment of a pre-subject, beamshaping filter in accordance with the present invention.

FIG. 7 is a schematic view of another embodiment of a filter assembly inaccordance with the present invention.

FIG. 8 is a pictorial view of a CT system for use with a non-invasivepackage inspection system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is described with respect to a radiographicimaging system such as the CT system shown in FIGS. 1-2 and 8 and thex-ray system shown in FIGS. 3-4. However, it will be appreciated bythose skilled in the art that the present invention is equallyapplicable for use with other radiographic imaging systems. Moreover,the present invention will be described with respect to the emission anddetection of x-rays. However, one skilled in the art will furtherappreciate, that the present invention is equally applicable for theemission and detection of other high frequency electromagnetic energy.

Referring to FIGS. 1 and 2, a “third generation” CT imaging system 10 isshown as including a gantry 12. The present invention, however, isapplicable with other CT systems. Gantry 12 has an x-ray source 14 thatprojects a beam of x-rays 16 through filter assembly 15 toward adetector array 18 on the opposite side of the gantry 12. Detector array18 is formed by a plurality of detectors 20 which together sense theprojected x-rays that pass through a medical patient 22. Each detector20 produces an electrical signal that represents the intensity of animpinging x-ray beam and hence the attenuated beam as it passes throughthe patient 22. During a scan to acquire x-ray projection data, gantry12 and the components mounted thereon rotate about a center of rotation24.

Rotation of gantry 12 and the operation of x-ray source 14 are governedby a control mechanism 26 of CT system 10. Control mechanism 26 includesan x-ray controller 28 that provides power and timing signals to anx-ray source 14, a gantry motor controller 30 that controls therotational speed and position of gantry 12, and filter assemblycontroller 33 that controls filter assembly 15. A data acquisitionsystem (DAS) 32 in control mechanism 26 samples analog data fromdetectors 20 and converts the data to digital signals for subsequentprocessing. An image reconstructor 34 receives sampled and digitizedx-ray data from DAS 32 and performs high speed reconstruction. Thereconstructed image is applied as an input to a computer 36 which storesthe image in a mass storage device 38.

Computer 36 also receives commands and scanning parameters from anoperator via console 40 that has a keyboard. An associated cathode raytube display 42 allows the operator to observe the reconstructed imageand other data from computer 36. The operator supplied commands andparameters are used by computer 36 to provide control signals andinformation to DAS 32, x-ray controller 28, and gantry motor controller30. In addition, computer 36 operates a table motor controller 44 whichcontrols a motorized table 46 to position patient 22 and gantry 12.Particularly, table 46 moves portions of patient 22 through a gantryopening 48.

Referring now to FIGS. 3-4, an x-ray system 50 incorporating the presentinvention is shown. The x-ray system 50 includes an oil pump 52, ananode end 54, and a cathode end 56. A central enclosure 58 is providedand positioned between the anode end 54 and the cathode end 56. Housedwithin the central enclosure 58 is an x-ray generating device or x-raytube 60. A fluid chamber 62 is provided and housed within a lead linedcasing 64. Fluid chamber 62 is typically filled with coolant 66 thatwill be used to dissipate heat within the x-ray generating device 60.Coolant 66 is typically a dielectric oil, but other coolants includingair may be implemented. Oil pump 52 circulates the coolant through thex-ray system 50 to cool the x-ray generating device 60 and to insulatecasing 64 from high electrical charges found within vacuum vessel 68. Tocool the coolant to proper temperatures, a radiator 70 is provided andpositioned at one side of the central enclosure 58. Additionally, fans72, 74 may be mounted near the radiator 70 to provide cooling air flowover the radiator 70 as the dielectric oil circulates therethrough.Electrical connections are provided in anode receptacle 76 and cathodereceptacle 78 that allow electrons 79 to flow through the x-ray system50.

Casing 64 is typically formed of an aluminum-based material and linedwith lead to prevent stray x-ray emissions. A stator 80 is also providedadjacent to vacuum vessel 68 and within the casing 64. A window 82 isprovided that allows for x-ray emissions created within the system 50 toexit the system and be projected toward an object, such as, a medicalpatient for diagnostic imaging. Typically, window 82 is formed in casing64. Casing 64 is designed such that most generated x-rays 84 are blockedfrom emission except through window 82. X-ray system 50 includes apre-subject filter assembly 86 designed to control an attenuationprofile of x-rays 84.

Referring now to FIG. 5, an x-ray generation and filtering assembly inaccordance with the present invention and at least applicable with theCT and X-ray systems described above is schematically shown. Assembly 87includes an x-ray source 88 that projects a beam of x-rays 90, or otherhigh frequency electromagnetic energy beam toward a subject (not shown).As will be described, beam 90 has a profile 92 that is tailored to atleast approximate physical characteristics, e.g. shape, of the subject.Attenuating the x-ray beam 90 prior to attenuation by the subject todefine profile 92 is a pre-subject, beam-shaping filter assembly 94.

Filter assembly 94 includes a pair of filters or filter components 96and 98 that generally mirror each other in shape and orientation. Inthis regard, each filter 96, 98 constitutes roughly one-half of thefilter assembly. As will be described below, in a preferred embodiment,the filters are not dimensionally equivalent. Notwithstanding thedifferences in sizes, each filter is defined by a base 100, 102, a tail104, 106, and a curved or arcuate portion 108, 110. In this regard,attenuation of x-rays by each filter is non-uniform across the filterbody. That is, since the base of each filter is thicker than the tail ofeach filter, the bases of each filter attenuate more x-rays than thetails of each filter. In one embodiment, the base of each filter has athickness of 30 mm and each tail has a thickness of 0.25 mm. The degreeof attenuation is function of the attenuation material used to fabricatethe filter and the relative thickness of each filter portion.

Each filter 96, 98 is operationally connected to a motor assembly 112,114, respectively. Each motor assembly receives control signals from acontroller and/or computer of the imaging system, that when received,causes each motor assembly to position a respective filter in the x-raybeam or path 90. In one embodiment, each motor assembly includes astepper motor, but it is contemplated that other types of motors may beused to position the filters. The motor assemblies 112, 114 are alsodesigned to re-position the filters independently throughout dataacquisition. In this regard, each filter may be separately anddynamically controlled or positioned to achieve a particular attenuationprofile 92 throughout data acquisition. Moreover, it is preferred thatboth filters are connected and controlled by a respective motorassembly. Additionally, one filter could be fixed and remain stationaryto the other filter. It is further contemplated that more than twofilters may be used.

In an exemplary embodiment, the distal end (relative to the x-raysource) of filter 96 is 117 mm from the x-ray source 88. The distal endof filter 98 is set at 148 mm from the x-ray source in this exemplaryembodiment. Additionally, in this exemplary embodiment, the base offilter 96 has a length along the x-axis of 45 mm, the tail has a lengthof 135 mm, and the connecting curved portion has a length of 24.9 mm. Incontrast, the base of filter 98 has a length in the x-direction of 53mm, the tail has a length of 168 mm, and the connected curved portionhas a length of 34.2 mm. The dimensions of each curved portion are setforth in the table below. One skilled in the art will readily appreciatethat the above dimensions are illustrative of only one of a number ofpossible embodiments. Curvature X, Y Coordinate Dimensions Filter 96 XFilter 96 Y Filter 98 X Filter 98 Y 0.00000 0.140964 0.00000 0.1409641.52658 0.277455 1.92109 0.277455 3.02431 0.736801 3.81409 0.7368014.48315 1.49686 5.66911 1.49686 5.89467 2.53118 7.47786 2.53118 7.251983.81159 9.23358 3.81159 8.54973 5.30908 10.9311 5.30908 9.78406 6.9945412.5666 6.99454 10.9524 8.83954 14.1378 8.83954 12.0536 10.8169 15.643610.8169 13.0874 12.9009 17.0839 12.9009 14.0545 15.0681 18.4596 15.068114.9562 17.2971 19.7722 17.2971 15.7946 19.5688 21.0238 19.5688 16.572021.8668 22.2169 21.8668 17.2910 24.1766 23.3544 24.1766 17.9543 26.486224.4391 26.4862 18.8075 27.9529 25.7168 27.9529 19.8335 28.7495 27.170528.7495 20.9281 29.2923 28.6963 29.2923 22.0739 29.6668 30.2769 29.666823.2688 29.9013 31.9104 29.9013 24.5186 29.9983 33.6029 29.9983

Motor assemblies 112, 114 axially and independently position filters 96,98, respectively, along the x-direction in the x-ray path so that thecollective attenuation of the filters defines a target attenuationprofile. In one embodiment, each motor positions a respective filter byextending and retracting respective piston assemblies 113 and 115. Oneskilled in the art will appreciate that other assemblies may be used toextend and retract the filters into and from the x-ray path. Based onthe positioning of the filters, the attenuation caused by filter 96 isadded to the attenuation caused by filter 98. Since each filter has acontour that defines a multiple thickness, the combined contourscollectively define a multitude of possible beam profiles. A particularbeam profile may therefore be selected from the multitude of possiblebeam profiles so that that the resulting beam profile is tailored to theparticular patient or subject. That is, filters 96, 98 may be positionedrelative to one another by their respective motor assemblies 112, 114 todefine a beam profile that substantially matches an approximate shape ofthe patient. Also, filters 96 and 98 are shown as at least partiallyoverlapping one another. It is contemplated, however, that the filtersbe positioned such that no overlapping occurs.

Shown in FIG. 6 is another embodiment of the present invention. Toreduce size constraints on the CT or x-ray system, filters 96 and 98, asshown in FIG. 6, are oriented with respect to one another such that thetail 104 of filter 96 is positioned proximate to the base 102 of filter98. Similar to the orientation of FIG. 5, a desired attenuation profilemay be formed by independently positioning filters 96 and 98 relative toone another. Additionally, the volume occupied by the orientation shownin FIG. 6 is effectively one-half of that required by the orientation ofFIG. 5. It should be noted that in the orientation shown in FIG. 6, itis preferred that filter 98 have a shape different from that of filter96 so that the x-ray path lengths are identical for both filters.Determining the appropriate shape may be achieved by determining thepath length for each fan angle on filter 96 and then locating(determining) the location of the filter boundary on filter 98 with thesame fan angle.

Referring now to FIG. 7, another embodiment of a filter assembly inaccordance with the present invention is schematically shown. In thisembodiment, filters 96 and 98 are constructed without tail portions 104and 106, respectively. In this regard, each filter 96 and 98 have a base100, 102 and a curved portion 108, 110. Replacing the tail portions is aretractable or stationary, and relatively thin attenuation plate 116. Inthe illustrated embodiment, plate 116 is fixed, but it is contemplatedthat plate 116 may be connected to a motor assembly that controls theposition of plate 116 in the x-ray path. Plate 116 also operates toprovide a minimum, non-zero amount of attenuation if filters 96 and 98are separated from one another, i.e. no filter overlap.

As described, filters 96 and 98 may be positioned in the x-ray path byrespective motor assemblies. The positioning of the filters 96 and 98may be set prior to the beginning of a scan and remained fixed duringdata acquisition, or filters 96 and 98 may be dynamically andautomatically re-positioned throughout the data acquisition process toachieve a desired or target attenuation profile. In either case, it ispreferred to carry out a scout scan of the subject to determine anoptimal beam profile for that particular subject. The scout scanpreferably gathers information relative to the subject's shape and size.One skilled in the art will readily appreciate that othercharacteristics may be taken into consideration when determining anappropriate attenuation profile. From this scout scan, a computer in theimaging system provides control signals to the respective motorassemblies, that when executed, causes the motor assemblies to positionthe filters relative to one another in a specified position and, ifapplicable, to re-position the filters during data acquisition. In thisregard, the intensity of the x-ray beam on particular anatomicallocations may be precisely controlled.

Referring now to FIG. 8, package/baggage inspection system 118 includesa rotatable gantry 120 having an opening 122 therein through whichpackages or pieces of baggage may pass. The rotatable gantry 120 housesa high frequency electromagnetic energy source 124 as well as a detectorassembly 126. A conveyor system 128 is also provided and includes aconveyor belt 130 supported by structure 132 to automatically andcontinuously pass packages or baggage pieces 134 through opening 122 tobe scanned. Objects 134 are fed through opening 122 by conveyor belt130, imaging data is then acquired, and the conveyor belt 130 removesthe packages 134 from opening 122 in a controlled and continuous manner.As a result, postal inspectors, baggage handlers, and other securitypersonnel may non-invasively inspect the contents of packages 134 forexplosives, knives, guns, contraband, etc.

The present invention is directed to a filter assembly for aradiographic imaging scanner that allows the x-ray beam profile to beadjusted along a continuum matched to the particulars of the subject tocontrol a dynamic range of x-ray flux and achieve optimum doseefficiency. Further, the x-ray beam may be controlled during dataacquisition to account for an off-centered subject. Moreover, imageartifacts are reduced because the filters are absent discontinuous edgesor mechanical interfaces. Additionally, manufacturability of the filtersis not unduly complex and implementation of the filters does not requireextensive changes in existing radiographic imaging system design.

Therefore, in accordance with one embodiment of the present invention, abeam shaping filter assembly is provided. The filter assembly includes afirst moveable filter having a non-uniform thickness and a secondmoveable filter independent of the first moveable filter and having anon-uniform thickness. Each filter is configured to be placed in a highfrequency electromagnetic energy beam for attenuation of the beam forradiographic data acquisition.

In accordance with another embodiment, a CT system is disclosed thatincludes a rotatable gantry having an opening to receive a subject to bescanned and a high frequency electromagnetic energy projection sourceconfigured to project a high frequency electromagnetic energy beamtoward the subject. A pre-subject filter assembly including a pair offilters and a scintillator array having a plurality of scintillatorcells wherein each cell is configured to detect high frequencyelectromagnetic energy passing through the subject are also provided.The CT system also includes a photodiode array optically coupled to thescintillator array and comprising a plurality of photodiodes configuredto detect light output from a corresponding scintillator cell and a dataacquisition system (DAS) connected to the photodiode array andconfigured to receive the photodiode outputs. An image reconstructor isprovided and connected to the DAS and configured to reconstruct an imageof the subject from the photodiode outputs received by the DAS. The CTsystem further includes a controller configured to independentlyposition each filter of the pair of filters in the high frequencyelectromagnetic energy beam so as to modulate the beam to have a profilethat substantially matches at least an approximate shape of the subject.

According to another embodiment, the present invention includes an x-rayfilter assembly having a first filter and a second filter. A first motorassembly is connected to the first filter and a second motor assembly isconnected to the second filter. The first and the second motorassemblies are configured to independently position a respective filterin an x-ray path to define an attenuation profile that substantiallyapproximates a target shape.

The present invention has been described in terms of the preferredembodiment, and it is recognized that equivalents, alternatives, andmodifications, aside from those expressly stated, are possible andwithin the scope of the appending claims.

1. A computed tomography (CT) system comprising: a rotatable gantryhaving an opening to receive a subject to be scanned in a subject area;a high frequency electromagnetic energy projection source configured toproject a high frequency electromagnetic energy beam toward the subjectarea; a pre-subject filter assembly including: a pair of filters, eachfilter defined by a base, tail, and curved portion connecting the baseto the tail, and wherein the pair of filters are arranged such that thecurved portion of one filter generally faces the high frequencyelectromagnetic energy projection source and the curved portion ofanother filter generally faces the high frequency electromagnetic energyprojection source, wherein the filters are moveable along a plane at aright angle to a central beam of x-rays emitting from the high frequencyelectromagnetic energy projection source; and a third filter having alength perpendicular to the central beam of x-rays, the length beinglonger than at least one of the first and second filters
 2. The CTsystem of claim 1 further comprising: a scintillator array having aplurality of scintillator cells wherein each cell is configured todetect high frequency electromagnetic energy passing through thesubject; a photodiode array optically coupled to the scintillator arrayand comprising a plurality of photodiodes configured to detect lightoutput from a corresponding scintillator cell; and a controllerconfigured to independently position at least one filter of the pair ofmoveable filters in the high frequency electromagnetic energy beam so asto modulate the beam to have a profile that substantially matches atleast an approximate shape of a subject.
 3. The CT system of claim 1wherein the pair of moveable filters mirror one another relative to thecentral axis of x-rays.
 4. The CT system of claim 1 further comprising acomputer programmed to cause application of a scout scan of the subjectand from the scout scan determine at least an approximate shape of thesubject.
 5. A method for conducting a CT scan of a subject using atailored profile of an x-ray beam in an imaging system, the imagingsystem including a detector array, a high frequency electromagneticenergy projection source, a pre-subject filter assembly including afirst filter, a second filter and a detector assembly, the methodcomprising the steps of: selecting a scan type; determining a desiredx-ray beam attenuation profile for a subject to be scanned; positioningthe first and second filters to move along a plane orthogonal to acentral beam of x-rays emitting from the high frequency electromagneticenergy projection source; and attenuating the x-ray beam by passingx-rays through the pre-subject filter assembly.
 6. The method of claim 5further comprising tailoring the x-ray beam to a desired x-ray beamattenuation profile based on the scout scan.
 7. The method of claim 5further comprising positioning a third filter to receive x-rays thatpass through at least one of the first and second filters.
 8. The methodof claim 7 wherein the third filter is retractable.
 9. The method ofclaim 7 wherein the third filter is fixed.
 10. The method of claim 5wherein selecting a scan type comprises the step of selecting one of abody scan and a head scan.
 11. The method of claim 5 wherein determiningthe desired x-ray beam attenuation profile comprises the step ofconducting a scout scan of the subject.
 12. The method of claim 11wherein conducting a scout scan comprises the step of gatheringinformation relative to a shape and size of the subject.
 13. The methodof claim 5 wherein positioning the first and second filters comprisesthe step of moving at least one filter of the first and second filtersto tailor the x-ray beam to the desired x-ray beam attenuation profile.14. The method of claim 5 wherein performing a scan of the subjectcomprises rotating the imaging system about a gantry within an imagingplane; emitting x-rays as a beam from a focal point at a source; passingx-rays through the pre-subject filter; passing x-rays through thesubject; receiving x-rays on the detector assembly; and reconstructingan image from detector assembly outputs.
 15. An x-ray filter assemblycomprising: a first filter having a curved portion and a second filterhaving a curved portion, the first and second filters arranged such thatthe first filter and the second filter are moveable along a plane thatis substantially perpendicular to a central beam of x-rays emitting froman x-ray source; a third retractable filter positioned to receive x-raysthat pass through at least one of the first and second filters; a firstmotor assembly connected to the first filter and a second motor assemblyconnected to the second filter.
 16. The x-ray filter of claim 15 whereinthe first and the second motor assemblies are configured toindependently position a respective filter in an x-ray path to define anattenuation profile that substantially approximates a target shape. 17.The x-ray filter assembly of claim 15 wherein the first filter has acontour different than that of the second filter.
 18. The x-ray filterassembly of claim 15 wherein each moveable filter is defined by a base,the curved portion, and a tail, and wherein the first and the secondmotor assemblies are further configured to position the first and secondfilters such that the tail of the first filter is proximate to the tailof the second filter.
 19. The x-ray filter assembly of claim 15incorporated into a CT system.
 20. The x-ray filter assembly of claim 15wherein the CT system includes a computer programmed to determine thetarget shape from a scout scan of a subject to be imaged.